Optical fibres are today used in numerous applications that span very diverse fields of optics. These fields include telecommunications, medicine, sensors, lasers, amplifiers and many others.
Double Clad Fibres for Laser and Amplifier Applications
About 10 years ago, a new family of optical fibres appeared, called double clad fibres (also known as double cladding fibres). Such fibres receive a large interest due to their potential for use in high power amplifiers and lasers. They consist of two waveguides embedded into each other; an inner and an outer guiding region. Typically, the inner guiding region is a single mode core for guiding signal light, whereas the outer region typically is a multi mode core, also called inner cladding (or pump core), for guiding pump light.
The term ‘double clad’ or ‘double cladding’ optical fibre is in the present context taken to refer to an optical fibre comprising at least two cladding regions extending in a longitudinal direction of the optical fibre, at least one of which may be used for propagating light, e.g. pump light, this cladding region therefore is also termed ‘a pump core’. The term is NOT intended to exclude the use of optical fibres comprising more than two such cladding regions. Different cladding regions are e.g. differentiated by different optical properties (such as refractive indices) of their background materials, a cladding region comprising micro-structural elements differing from a cladding region NOT comprising any, cladding regions comprising different micro-structural elements differing from each other (the micro-structural elements of the respective cladding regions differing in any property having an influence on the propagation of light at the appropriate wavelength, e.g. by a different size of the micro-structural elements (if not interspersed), by different materials of the micro-structural elements (e.g. voids, solid or liquid), regularly arranged vs. irregularly arranged, etc.), etc.
A typical use for double cladding fibres is to efficiently convert low quality, low brightness light from e.g. semiconductor lasers (lasers providing pump light) to high quality, high brightness light (signal light). This can be done for both laser and amplifier configurations. For laser configurations the signal light is generated through stimulated emission and within a cavity (typically formed from fibre Bragg gratings and/or external mirrors). For amplifier configurations, a seed signal is coupled to the single mode core and amplified through stimulated emission.
Brightness is defined as optical power per solid angle per unit area, also termed luminance and measured in the SI-units of Candela/m2 or W/steradian/m2. For multi mode fibres, conservation of brightness means that the NA multiplied with the waveguide diameter is a constant before and after the coupling/conversion.
The brightness conversion can be implemented by doping the core with an optically active material, e.g. a rare earth dopant and pumping this with pump light, e.g. multi mode light. The rare earth atoms will absorb the pump light and re-emit the energy at lower photon energies. Since the emission will happen through stimulated emission, this light will be guided in the doped core. Typically single mode operation is preferred, but multi-mode operation is also relevant.
This conversion method can be very efficient (up to around 80%) and the brightness can be improved by more than a factor of 100. Such light sources are often used as popular alternatives to high brightness solid state lasers, since they are less bulky and far more efficient.
Double clad fibres can be provided in various types (micro-structured as well as non-micro-structured fibres) that are all relevant to the present invention. These types include all-glass fibres (see e.g. Wang et al., Electronics Letters, Vol. 40, No. 10, 2004), polymer clad fibres (see e.g. Martinez-Rios et al., Optics Letters, Vol. 28, No. 18, 2003) and photonic crystal fibres (see e.g. WO 03/019257)
Photonic Crystal Fibres.
Photonic crystal fibres (PCFs) have recently emerged as an attractive class of fibres, where various properties may be tailored in new or improved manners compared to conventional (solid, non-micro-structured) optical fibres. PCFs are generally described by Bjarklev, Broeng, and Bjarklev in “Photonic crystal fibres”, Kluwer Academic Press, 2003. The fabrication of PCFs is e.g. described in chapter IV, pp. 115-130.
In recent years, PCFs have been developed to also show double cladding features. Here, a ring of closely spaced air holes (air-clad) will define the multi mode inner cladding. Fibres with air-cladding and their fabrication are e.g. described in U.S. Pat. No. 5,907,652 and WO 03/019257 that are incorporated herein by reference. The Numerical Aperture (NA) of PCFs can take values from below 0.2 all the way up to more than 0.8, although typical values lie around 0.6.
Coupling to Double Clad Fibres Using Bulk Optics.
A common problem in fibre optics is to launch light into a fibre efficiently.
Often the source of light and the fibre to couple into have different divergence angles (numerical aperture (NA)) and spot/core sizes. A specific problem is to launch light from a pump-diode-laser with a large spot size and relatively low numerical aperture into a double clad fibre laser with a small area and large numerical aperture.
The traditional method of solving this problem is to use bulk optics. An example can be seen in FIG. 1, where pump light from a single source, for example a fibre 10 delivering a pump light, is to be coupled into a single end of a PCF 11 (a PCF chosen only as an example of a double clad fibre). The first (slow) lens 12 collimates the light 13 from the pump fibre, whereas the second (fast) lense 14 focuses the light into the inner cladding of the PCF. This approach has the disadvantage that only one pump fibre can be used. Also, such a solution typically has only a coupling efficiency of 80-90%, has high reflections, is sensitive to mechanical drift and instability and sensitive to contamination. Finally, such solution makes packaging design for a commercial device complicated and expensive.
The solution of bulk optics has a number of problems. One problem is related to difficulties in achieving coupling with low loss. Another problem is to achieve good coupling for a wide range of wavelengths. A third problem is mechanical stability. Fabrication of devices using bulk optics is also relatively complicated. Furthermore, reflection from the multiple glass surfaces may degrade performance of the system.
Coupling to Double Clad Fibres Using a Tapered Fibre Bundle.
In order to couple light from multiple pump lasers to a double clad fibre, a common approach is to use a coupler known as a so-called tapered fibre bundle (also known as fused, tapered fibre bundles). Such couplers have been developed by a number of optical component supplier companies, such as ITF, SIFAM, OFS, JDSU and Nufern—and are described in for example U.S. Pat. No. 5,864,644 or in U.S. Pat. No. 5,935,288.
An example of a tapered fibre bundle is shown in FIG. 2. Several fibres 20 are bundled together and heated to temperatures near melting and tapered 21. Using a taper, light from each fibre that delivers pump light (pump fibre that typically supports an NA between 0.15 and 0.22) will merge and as the fused region tapers down in dimensions, the NA slowly (adiabatically) increases (typically up to around 0.45 or even higher). The tapered region is typically surrounded directly by air—resulting in an unprotected silica-glass interface. The fused, tapered end of the coupler is typically spliced to a double clad fibre.
The problem with fused, tapered fibre bundles is that it is difficult to couple pump light efficiently into a high NA double clad fibre (NA higher than 0.3). It is thus an object of the invention to provide a fibre coupler for coupling two or more light sources into a multi-clad (e.g. double clad) optical fibre, the coupler being improved with respect to the prior art fibre couplers. It is a further object to provide a fibre coupler which is improved with respect to low loss.
A further problem of fused, tapered fibre bundles is that it is difficult to package these, since the tapered region comprises an uncoated waveguide region. This region typically being solid glass surrounded by air (the waveguide structure for the pump light in the tapered region) that is fragile and difficult to package. It is thus an object of the invention to provide a component for pump multiplexing that is less fragile and simpler to package.
Coupling to Double Clad Fibres Using a Tapered Fibre Bundle with Signal Feed-Through.
The bundle of fibres 20 may also comprise a single mode fibre (typically placed in the centre of the bundle of fibres 20). Such a fibre may serve for feed-through of signal light. This component is known as an all-fibre signal-pump multiplexer and is typically used in fibre amplifier configurations. The single mode fibre comprises a single mode core and is typically a single clad fibre. For these signal-pump multiplexers also the single mode fibre is tapered. Such signal-pump multiplexers may be used for co- or counter-propagating pump light.
A further problem of fused, tapered fibre bundles is that signal light can be reflected back into the pump delivery fibres—causing damage to the lasers that deliver the pump light. One way of reducing the amount of reflected signal light is to use the signal-pump multiplexer in a configuration, where pump and signal light is counter-propagating. However, even in such a configuration, problems have been found for commercial available signal-pump multiplexers for signal average powers levels of around 10 mW (the exact level depends on the quality of the multiplexer and the specifications of the signal light (e.g. continuous wave, pulse, pulse duration)). It is thus an object of the invention to provide a component for signal-pump multiplexing that has a low reflection of signal light into pump delivery fibres.